modesetting: Disable atomic support by default

The atomic driver has issues with modesetting when stealing
connectors from a different crtc, a black screen when doing rotation
on a different crtc, and in general is just a mapping of the legacy
helpers to atomic. This is already done in the kernel, so just
fallback to legacy by default until this is fixed.

Please backport to 1.20, as we don't want to enable it for everyone
there. It breaks for existing users.

The fixes to make the xserver more atomic have been pending on the
mailing list for ages.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110375
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110030
References: !36 (commits)


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>